Magnetic resonance imaging apparatuses are apparatuses for obtaining physical and chemical information of an object of measurement by irradiating a radio frequency magnetic field of a specific frequency on the object of measurement placed in a static magnetic field to induce magnetic resonance phenomenon. The magnetic resonance imaging (MRI) method currently widely spreading is a method of imaging hydrogen nucleus density, difference of relaxation time, or the like, which differs depending on type of body tissue, by mainly using magnetic resonance phenomenon of protons in water molecules. Difference of tissues can be thereby imaged, and thus it is highly effective for diagnosis of diseases.
On the other hand, MRS and MRSI are methods of separating magnetic resonance signals for every molecule (metabolite) on the basis of difference in resonant frequency thereof (chemical shift) caused by difference of chemical bonds in the molecule, and measuring density, difference of relaxation time, or the like for every molecular species. MRS is a method of observing molecular species in a certain selected special region, and MRSI is a method of imaging every molecular species. The atomic nuclei used as the object include those of 1H (proton), 31P, 13C, 19F, and so forth.
Major metabolites existing in human bodies and detectable by proton MRS or proton MRSI utilizing protons as the objective nucleus species (henceforth referred to simply as MRS and MRSI) include choline, creatine, N-acetylaspartate (NAA), lactate, and so forth. It is expected to perform non-invasive stage determination or early diagnosis, and diagnosis of malignancy of metabolic disorders such as cancers, on the basis of amounts of such metabolites.
MRS and MRSI are applicable not only to measurement of metabolite densities, but also to thermometry in living bodies utilizing difference of resonant frequencies of water and metabolite. It is known that the resonant frequency shift of water depends on temperature, and the temperature coefficient of the shift amount is −0.01 ppm/° C. (for example, Non-patent document 1). It is also known that resonant frequencies of such metabolites as NAA do not change in the temperature range under the physiological environment. There is a technique for measuring a temperature in a living body on the basis of the difference of resonant frequencies of water and metabolite utilizing the above characteristics (refer to, for example, Non-patent document 2).
The temperature information is calculated by fitting with a model function, for example, as follows. First, water and metabolite (the following explanation will be made for NAA as an example) are measured individually or simultaneously. Then, the measured data are subjected to the Fourier transform in the time direction to obtain spectra, respectively. Fitting of the measured peak regions (spectral peaks) of water and NAA is performed by using, for example, the Lorenz type function represented by the following equation (1).
                    [                  Equation          ⁢                                          ⁢          1                ]                                                                                  L            i                    ⁡                      (            v            )                          =                                                                              a                  i                  2                                ⁢                                  I                  i                                                                              a                  i                  2                                +                                  4                  ⁢                                                            (                                              v                        -                                                  v                          i                                                                    )                                        2                                                                        ⁢            cos            ⁢                                                  ⁢                          ϕ              i                                +                                                    2                ⁢                                  a                  i                                ⁢                                                      I                    i                                    ⁡                                      (                                          v                      -                                              v                        i                                                              )                                                                                                a                  i                  2                                +                                  4                  ⁢                                                            (                                              v                        -                                                  v                          i                                                                    )                                        2                                                                        ⁢            sin            ⁢                                                  ⁢                          ϕ              i                                +          c                                    (        1        )            In the equation, μ represents frequency, Li represents signal intensity, νi represents resonant frequency of an objective substance, a1 represents half-hand width of spectral peak, Ii represents height of spectral peak, φi represents phase, and c represents a constant term. Fitting of the measured spectral peaks of water and NAA is performed with the function of the equation (1) to obtain the resonant frequencies of water and NAA, respectively, as resonant frequency νi, which is a fitting parameter. Then, difference of resonant frequencies of water and NAA is calculated, and temperature is calculated in accordance with, for example, the temperature conversion equation described in Non-patent document 2.